An FA (Factory Automation) system includes a control target device and a programmable controller that performs a predetermined arithmetic operation using a state of the control target device as input data and that outputs operating conditions of the control target device as output data. The FA system enables real-time control over the control target device by providing communication units in the programmable controller and the control target device, respectively and connecting the communication unit to each other via a network. That is, the communication unit provided in the control target device functions as a slave station and that provided in the programmable controller functions as a communication management station controlling data transmission from the slave station. The communication management station receives data from the slave station and periodically performs processes of calculating data for controlling the control target device using the received data and of transmitting the obtained data to the slave station. At this time, the communication management station controls a timing of transmitting data from each slave station so as to ensure real-time data communication.
In such FA systems, one master station is connected to a plurality of slave stations, and when the number of master stations is one, the master station normally performs network management. However, when a plurality of master stations and a plurality of slave stations managed by each master station are present in the same network segment, any one of the master stations plays a role of the communication management station (network management station). In this case, one master station and a plurality of slave stations managed by the master station configure one group and thus a plurality of groups are present in the same network segment. With this configuration, data communicated in each group is transmitted by multicast communication, thereby making it possible to distinguish data of one group from data of the other groups (see, for example, Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-open No. H4-343543